N-alkylacrylamides, such as N-isopropyl, N-tert-butylacrylamide and N-n-octylacrylamide, are of important classes of monomers for the synthesis of polymers, which are useful in applications such as sizing agents, rheology modifiers and water soluble polymers. Amongst these, N-isopropylacrylamide has assumed significant importance owing to the commercial importance of its polymer. Poly(N-isopropylacrylamide) has been widely studied for its novel thermal behaviour in aqueous media [Schild H G. Progress in Polymer Science. 17, 163 (1992)] and possesses inverse solubility upon heating, a property contrary to the behaviour of most polymers in organic solvents under atmospheric pressure near ambient temperature. Its macromolecular transition from a hydrophilic to hydrophobic structure occurs at what is known as lower critical solution temperature (LCST). Experimentally, this temperature lies between 30-35° C., the exact temperature being a characteristic of the microstructure of the polymer. At molecular level, poly(N-isopropylacrylamide) has been used in many forms including single chain, macroscopic gel, microgels, latex, thin film, membrane, coating, and fibres.
N-alkylacrylamides (alkyl=C6 to C18) are also widely used as comonomers for the preparation of hydrophobically modified polyelectrolytes [Glass J E, Polymers in aqueous media:performance through association. ACS symposium series, 223. Washington: American Chemical Society, 1989 and Shalaby S W, McCormick C L, Glass J E. In: Shalaby S W, McCormick C L, Glass J E, editors. Water soluble polymers: synthesis, solution properties and applications. ACS symposium series 467. Washington: American Chemical Society, 1991]. These polymers consist of a water-soluble backbone containing a small proportion of hydrophobic groups (<3 mol %) usually in the form of pendant side chains or terminal groups. In aqueous solution the hydrophobic groups aggregate to minimize their exposure to water and, thereby, form hydrophobic microdomains in a fashion analogous to that of surfactants above their critical micelle concentration. Above a certain concentration (Cag), intermolecular hydrophobic interactions lead to the formation of a three dimensional network of polymer chains resulting in an increase in the apparent molecular weight and, consequently, a substantial viscosity enhancement.
Copolymers of N-alkylacrylamides with various other monomers are also finding diverse applications. For example, poly(N-dodecylacrylamide-co-N-methyl-4-vinyl pyridinium Na) is reported to be a useful as salt resistant viscosity builder [D. Christine, B. Alain and L. Pierre, Macromol. Symp. 102,233 (1995), D. Christine, B. Alain, B. Fransis and V. M. Laure, Polymer 36, 2095 (1995)], poly (N-stearoyl acrylamide-co-2-(3-acrylamidopropyl) dimethyl aminoethyl isoproply phosphate) is used as phosphatidylcholine analogous material [W Yenfeng, C. Tianming, K. Masaya and N. Taiao, J. Polym. Sci. Chem. Edn. 34, 449 (1996)], poly (N-tert-octylacrylamide-co-N-alkylacrylamide) has been employed as a thickener in cosmetics [J. Mondet and B. Lion Eur. Pat. Appl. EP 494,022] and poly (N-octylacrylamide-co-3-acrylamido-3-methyl butanoate Na) has been used for for oil recovery applications [A. Kitagawa and T Koichi, Jpn. Kokai Tokkyo Koho JP 07,188,347].
N-alkylacrylamides are, thus, a useful class of monomers. In order to meet the growing demand and new applications of N-alkylacrylamides, various methods have been developed for their synthesis.
Some of these methods for the synthesis of N-alkylacrylamides include (1) reaction of acryloyl chloride with alkyl amine; (2) pyrolysis or thermal decomposition of carboxylic acid amides, and (3) reaction of iso-olefins with nitriles.
In the first method N-alkylacrylamides are prepared by reacting acryloyl chloride with the corresponding alkyl amines in the presence of an acid quencher i.e. triethyl amine at 0° C. [C. G. Overberger, C. Frazier and J. Mandehman, J. Am. Chem. Soc. 75, 3326 (1953), J. Lal and G. S. Trick, J. Polym. Sci. A2, 4559 (1964), E. F. Jr. Jordan, G. R. Riser and B. Artymyshyn, J. Appl. Polym. Sci. 13, 1777 (1969), K. J. Shea, G. J. Stoddard, D. M Shavelle, F. Wakui and R. M. Chaote, Macromolecules 23, 4497 (1990)].
In the second method N-alkylacrylamides are prepared by amidation of bicyclic carboxylic acids followed by the thermal decomposition of the carboxamide. For example, the reaction of dimethylamine with bicyclo [2.2.1]hept-2-ene-2-carboxylic acid in an autoclave gave N,N-dimethyl bicyclo [2.2.1]hept-2ene-2-carboxylic acid which was subjected to thermal decomposition at 200° C./vacuum to give N,N-dimethyl bicyclo [2.2.1]hept-2-ene-2-carboxamide [A. Ohshima and K. Tsubashima Jpn. Tokkyo Koho 7909 170, A. Oshima, K. Tsubashima and N. Takahashi Ger. Offen. 2,217,623].
In method (3) N-alkylacrylamides are prepared by reacting acrylonitrile with various iso-olefins. It is also known to synthesise N-tert-octylacrylamide by reacting acrylonitrile with 2,4,4-trimethyl-1-pentene at 40° C. for 3 hours using 65% H2 SO4 as solvent [T Takada, Y Kawakatsu, T Mihamisawa and K Hara, Japan Kokai-7391011].
A method for the preparation of N-alkylamides using 60% H2SO4 and cation exchange resin as catalysts have been disclosed [S. Sivaram, N. Kalyanam, Ind. pat. 158395 A and S. Sivaram, N. Kalyanam Ind. Pat. 158038].
The above methods are beset with many disadvantages. Method (1) cannot be used for preparation of higher N-alkyl acrylamides (where alkyl chain length>C18) as alkylamines with>18 carbon atoms are not readily available. Besides, the method uses acryloyl chloride, which is an expensive and hazardous reagent and requires disposal of large quantities of chloride as waste.
Methods of type (2) suffer from the drawbacks of high temperatures, high vacuum and tedious work up procedures. This method is also applicable generally to only alkyl amides with small alkyl chain lengths. The reaction of olefins with nitriles is the most suitable general method for the synthesis of N-alkyl acrylamides and has been widely practiced. However, this method is restricted by the availability of suitable iso-olefins (isobutylene, 2,4,4-trimethyl-1-pentene etc) with carbon numbers less than eight or twelve.
It is therefore important to devise methods for the synthesis of N-alkylacrylamides which overcome the disadvantages of the prior art discussed above as well as create new N-alkylacrylamides in view of their growing importance in various fields of technology.